1. Field of the Invention
This invention relates to a camera having an autofocus function, independent of a photographing optical system, which includes optical elements using synthetic resin as material.
2. Description of Related Art
Synthetic resin is less expensive than glass and is easy to shape. Thus, the use of such synthetic resin facilitates the configuration of an aspherical lens and the improvement of imaging performance of an optical system. In recent years, optical apparatuses, such as cameras, have been required, to have an imaging optical system that includes optical elements which are constructed of synthetic resin having such excellent features. The optical elements constructed of synthetic resin are available in a single lens type in which a lens element is used alone and a compound lens type in which a lens element comes in close contact with a glass lens. However, the optical element constructed of synthetic resin, has particularly the defect that its imaging position is shifted.
In order to meet this problem, a technique is known that a change in temperature is determined by variation in electric resistance and thereby the shift of the imaging position is corrected. Another technique is also known that the shift of the imaging position caused by a temperature or humidity change is controlled by a lens design in such la way that the refracting power of a synthetic resin lens is lessened or at least two synthetic resin lenses are used and their refracting powers are set to minimize the shift of the imaging position caused by the temperature or humidity change in the entire optical system.
Further, a so-called TTL type focus detecting system is available in which a beam of light coming from an object, after passing through an Imaging optical system, is made incident on a light-receiving element, and thereby the imaging position is detected, together with the shift of the imaging position due to the temperature or humidity change.
A focus detecting system that is provided such that it is independent of the photographing optical system, on the other hand, as proposed by Japanese Patent Publication No. Hei 6-90356, is such that the defect that the imaging position is shifted by the temperature change is obviated by placing a member which expands or contracts according to temperature in a focus detecting device to produce variation in range measurement.
The first technique, mentioned above, of determining the change of temperature or humidity by the variation of electric resistance to thereby correct the shift of the imaging position has the following problems. Since the time required for the change of the shape or refractive index of the synthetic resin lens is greater than that required for the change of ambient temperature or humidity, it is difficult to measure only the ambient temperature or humidity to thereby predict the shift of the imaging position with accuracy. Moreover, there is no means which can be incorporated in a camera to measure the change of Humidity easily and inexpensively, and thus the change of humidity cannot be accurately followed. Consequently, it has been Impossible that means for measuring the change of humidity is incorporated in the camera to correct the shift of the imaging position caused by the change of humidity.
The second technique of minimizing the shift of the imaging position caused by the temperature or humidity change in the lens design involves the following problems. If the lens design is made to minimize the shift of the imaging position caused by the temperature or humidity change, the degree of freedom for setting the refractive index of the synthetic resin lens will be eliminated and an effective design cannot be prepared. For systems with at least two synthetic resin lenses, two kinds of synthetic resin are in general use to correct for chromatic aberration. In this case, however, the reaction of synthetic resin to the change of temperature or humidity varies with the kind of synthetic resin used for the lenses, and thus it cannot be ensured that the shift of the imaging position by the temperature or humidity change is always suppressed.
A general property of a lens constructed of synthetic resin is that the lens is liable to undergo the influence of a change in temperature. When the temperature is raised, for example, the radius of curvature and thickness of the lens are increased and the refractive index thereof is decreased. On the other hand, the influence of a change in humidity on the lens cannot also be neglected. For example, a in humidity rise causes the radius of curvature and thickness of the lens to be increased and the refractive index thereof to be also increased.
With the lens constructed of such synthetic resin, it is therefore difficult to ensure that the shift of the Imaging position caused by either the temperature or humidity change is kept to a minimum.
The TTL type focus detecting system, by contrast, allows the above problems to be solved. This system, however, makes use of a beam of light from an object, and thus requires circuits for performing a high grade of signal processing because the amount of light emanating from the object and its distribution change with objects, such as flowers, persons, and backgrounds. Moreover, system requires, to make two images coincide as a focusing mechanism, the system requires two light-receiving element arrays, two reimaging optical systems, and a complicated signal processing system for determining whether the two images coincide, and path splitting or switching means for splitting light from the photographing optical system to introduce the light into the reimaging optical systems. Thus, the TTL system, because of its many parts, has the disadvantages that the manufacturing costs are high and the camera body is bulky. Furthermore, since the system requires an image equivalent to that formed on a film, it is difficult to apply the system to the so-called lens shutter camera in which an aperture stop and a shutter are integrally constructed. That is, the system, although applicable to a single-lens reflex camera, is unsuitable for the lens shutter camera which is inexpensive and compact.
There is an example which is applicable to the lens shutter type, as the camera disclosed in Hei 6-90356 mentioned above, but this proposal encounters the problems that (1) accommodation for the humidity change is impossible, (2) the degree of freedom for optical design is not obtained, and (3) accommodation for a change with the passage of time is Impossible (where two or more kinds of materials are in particular used in the photographing optical system).